URANIUM-PRINTING PROCESS. A sheet of paper is ftrst rendered sensitive to light by immersing it in a strong solution of a salt of the peroxide of uranium, (the nitrate is probably the best). It is then dried, and exposed under a negative to clirect light for about the same time as an ordinary sun-print upon a chloride of silver paper. A very faint visible image is thus obtained, which is perceived by holding the paper ag,ainst the light. The print is then placed either in a weak solution of chloride of gold, or in a strong solution of aceto-nitrate of silver. In the former case a pic ture is obtained of a purple inky tint, and in the latter case of a chocolate brown tint. The print is then washed in abundance of water, several times renewed, and the operntion is complete.
The theory of the process appears to be as follows :— The uranium persalt is reduced by light to a protosalt, which, when the print is placed in the gold or silver developer, becomes again oxidized, and the gold or silver xeduced, either to a purple substance in the case of gold, or a brown substance in the case of silver. The redundant chemicals are then removed by washing, no
fixing agent being necessary.
The principal objection to this process appears to be the difficulty of obtaining good surface vigour, and fine definition ; there is also the fear of the lights becoming discoloured in consequence of the imperfect removal of the chemicals from the paper. Since no hyposulphite of soda is employed in fixing, the prints may be con sidered much more permanent than ordinary sun-prints that have ,been fixed and toned by hypo and gold.
-Uranium prints developed with silver may be intensified by im mersing them in a solution of protosulphate of iron acidified with acetic acid ; but the lights of the picture are very liable to become discoloured if no fixing agent, such as hypo or cyanide, be employed.
The uranium printing process is identical in principle with the Chrysotype process of Sir John Herschel, published in 1842 ; q. v.